1. Technical Field
The present disclosure relates to methods for preparing metal organic frameworks as sensors for the quantitative detection of analytes, particularly CO2.
2. Description of Related Art
The detection of carbon dioxide within mixtures of gases has proven difficult owing to the presence typically of competing oxygen, carbon monoxide and water vapor. It stands to reason, therefore, that the clear benefits of having robust and inexpensive devices to provide a quantitative analysis of CO2 concentrations in admixture with other gases provides more than enough impetus for the continued development of such devices. Much of the present sensing technology depends largely upon spectroscopic methods that become unreliable when the mixture of gases contains spectroscopically similar resonances. The complication and expense of fabricating the necessary devices for the detection of CO2 in applications where they could be most useful makes these devices highly sought after in their own right. For example, in the medical arena, new sensing technologies could improve human health by enabling the analysis of human breath when a patient is showing clinical signs of hypercapnia. Likewise, in the field of occupational safety, the buildup of CO2 emissions in the form of “blackdamp” in the mining and petroleum industries can be hazardous, particularly as we progress more and more toward coal liquefaction and the extraction of natural gas from shale oil deposits. Here, we describe a method to detect carbon dioxide within CO2/N2 and CO2/air mixtures using a recently described metal organic framework5 (MOF) composed of cyclodextrin (CD) and alkali metal (Group1A) cations.
Smaldone et al. (Angew. Chem., Int. Ed. 2010, 49:8630-8634) reported of a new cyclodextrin derived material called CDMOF-2 that exhibits strong but reversible binding of carbon dioxide. Though the authors were able to crudely demonstrate a colorimetric response of CDMOF-2 in the presence of CO2 by taking advantage of the unique chemistry that occurs within this highly porous material (Gassensmith et al., J. Am. Chem. Soc. 2011, 133:153121-5315), this response is by no means sufficient for practical quantitative analysis. This highly porous material belongs to a rapidly growing family of MOFs that are highly crystalline materials that are well structured chemically with building blocks—typically clusters of metal ions (components of the nodes) and rigid organics ligands (components of the extendable structural frameworks). Important features of MOFs are their (i) highly ordered nanoporosity, (ii) their large internal surface area, and (iii) the possibility of modifying their organic ligands post-synthetically. Accordingly, MOFs have been evaluated as potential nanoporous materials for applications in chemical separations, gas adsorption heterogeneous catalysis, ion exchange, drug delivery, ionic conduction, and sensing. More specifically, MOFs are receiving a lot of attention as a method for CO2 sequestration and colorimetric sensing.
No instances of conductance-based MOFs for CO2 detection have been reported in the literature. The need for an electrochemical means of sensing CO2 comes, in part, from an emerging environmental requirement to monitor concentrations at and near high volume, emission point sources, and from the limitations in present state of the art technologies. Although chemiresistive metal oxides and semiconducting field effect transistors have been thoroughly investigated, they have a limitation to their application—namely, from reaction with ambient oxygenated species absorbed on the oxide surface. In order to circumvent this limitation, semiconducting oxide sensors usually operate at temperatures in excess of 200° C. As an alternative, MOFs have been shown to be an excellent choice for sensing analytes at relatively low temperatures.
The majority of previously-designed MOFs for the selective uptake of CO2 have not exhibited reversible chemisorption desorption of carbon dioxide.